What is Ś in Sumero-Akkadian cuneiform?

Question

In a few different places (1) (2), ORACC lists Ś (S with acute accent, U+015A) as a Unicode character used for Sumero-Akkadian cuneiform transcription.

However, I've never seen this letter used (in ORACC or anywhere else), and am not sure what it would mean—to my knowledge, Sumero-Akkadian cuneiform only ever distinguished four sibilants (S Z Ṣ Š), and even those had some overlap between them.

So, what is this Ś in Sumero-Akkadian cuneiform?

(P.S. I know the letter Ś is sometimes used in transcribing Ugaritic cuneiform, and in discussions of Proto-Semitic, but if ORACC wanted to cover those they'd need a whole lot more glyphs in their Unicode tables: Ḥ, Ġ, etc. It seems notable that this is the only glyph in 1 that I don't recognize from either Sumerian or Akkadian.)

Answer 1

ś is the conventional transliteration for Hebrew שׂ ( śīn ), and is used also for its Semitic source, now more usually transcribed as s₂. It is believed that Old Akkadian (at least) still retained the Semitic distinction of s₁, s₂ and s₃ and used different signs for syllables containing each of these. This is reflected by the transcription of those signs.

Answer 2

Hasselbach 2005 discusses the use of ś in OAkk transcription, which she says was introduced by von Soden, but ultimately rejects it as unclear.

According to her, modern (post-Gelb) scholars generally reconstruct three sibilant "phonemes" for Sargonic Akkadian, which are consistently written differently. There may have actually been more phonemic contrasts here, but if so they were hidden by the writing system.

In von Soden's transcription:

• ś (Hasselbach's s) comes from Proto-Semitic *š and *ś, and is written with the Sumerian S signs
• š (Hasselbach's θ) comes from Proto-Semitic *θ, and is written with the Sumerian Š signs
• s (Hasselbach's z) comes from Proto-Semitic *s, *z, *s', *ð, *θ', and ś', and is written with the Sumerian Z signs

In other words, ś represents the reflex of Proto-Semitic *š and *ś when written with the S signs in Sargonic Akkadian. In Classical Akkadian, the reflex of these two PS consonants was written with the Š signs, and is thus transcribed as š instead.

Even von Soden later seems to have regretted this particular convention, writing in 1995 (my translation):

In other words, the Old Akkadian phoneme ś is a phantom, originating in a confusion between graphemes and phonemes. There are probably no sibilants other than those in Old Babylonian; they are just divided up and written differently.